This invention relates to a protective ring for a fan protective casing of a gas turbine engine in the form of a full containment or in the form of a penetration containment with trapping layer.
During take-off with maximum power demand, the fan of an aircraft jet engine is subject to the highest loads which, under extreme conditions, may lead to a failure of the fan blades, for example if large birds or objects on the runway are ingested by the fan and strike the fan with high kinetic energy. If such an event occurs, any broken-off fan fragments must absolutely be precluded from breaking through the engine casing in order to avoid serious consequences to the aircraft itself. In order to avoid destruction of the engine, a special type of fan protective casing or protective ring, also referred to as a containment, is provided around the particularly endangered casing area on the outer circumference of the fan.
Depending on their design and function, distinction is made between two types of protective casings, namely the full containment and the penetration containment.
The full containment is designed such that a failed fan blade or a blade fragment, while being allowed to cause deformation of the protective ring, will not be able to break through the protective ring. It will finally be discharged from the engine via the by-pass duct without bringing about further damage. This solution, while being functionally favorable, has the disadvantage of high weight, resulting from the compact metal ring of which such a protective casing consists.
A known design of penetration containment comprises a thin-walled metal ring through which the failed fan fragment can break, but with the fan fragment being retained by a weave arranged on the outer circumference of the metal ring, the so-called trapping layer. The penetration containment, which is definitely lighter than and in this respect, superior to the full containment, is, however, problematic in that, upon breakthrough, the damaged fan wheel, which now runs out of balance, will generate vibrations in the engine casing. This can result in an increase of the cracks existing at the point of breakthrough on the metal ring and in corresponding consequential damage.
In the case of a penetration containment described in Specification U.S. Pat. No. 4,699,567, a rigid, thin-walled cylinder made of light metal or fiber-reinforced resin is initially covered with woven fiber-material layers consisting of single material pieces, with the inner layers so formed being wrapped with further fiber-material layers of continuous strip length. The outer surface of the containment is covered with an impermeable metallic or non-metallic protective layer. If a flinging blade fragment breaks through the inner ring (metallic protective ring), it will on its route be first enclosed and padded by the inner fiber-material pieces and finally be caught by the outer, continuous fiber-material layers, these absorbing the impact energy by elastic deformation. A further propagation of the damage existing in the rigid protective ring of the containment can, however, not be avoided.
In a fan casing structure known from Specification U.S. Pat. No. 5,486,086, an inner, rigid safety cylinder (protective ring), which surrounds the fan blades at a certain distance, is provided on the outer surface with a plurality of ribs extending in the longitudinal and circumferential direction, these ribs being intended to prevent the propagation of cracks beyond the rib-reinforced material zones.